1. Field of the Invention
The present invention relates to vacuum pumps used in semiconductor manufacturing apparatus and so on, and more particularly, the present invention relates to a vacuum pump which reduces a damaging torque produced when a rotor rotating at high-speed crashes into a screw stator or the like.
2. Description of the Related Art
In a process such as dry etching, chemical vapor deposition (CVD), or the like performed in a high-vacuum process chamber in semiconductor manufacturing step, a vacuum pump such as a turbo-molecular pump is used for producing a high vacuum in the process chamber by exhausting gas from the process chamber.
FIG. 5 is a vertical sectional view of a conventional vacuum pump. In the vacuum pump, a pump case 1 is provided with a gas suction port 1-2 at the top portion thereof. The pump case is in communication with a process chamber 17 by connecting the flange 1a to the process chamber 17 with fastening bolts 15.
The vacuum pump fixed to the process chamber 17 is provided with a rotor shaft 12, a rotor 2 and rotor blades 4, and the rotor shaft 12 rotates together with the rotor 2 and the rotor blades 4 when the vacuum pump is in operation. Also, the vacuum pump is also provided with stator blades 5, and a screw stator 7 fixed therein. Gas molecules in the process chamber 17 is exhausted out from the gas exhaust port 1-3 passing through the gas suction port 1-2 and then the pump case 1 by the interaction between the rotor blades 4 rotating at high-speed and the stator blades 5 and the other interaction between the rotor 2 at high-speed rotating and the screw stator 7 having thread grooves 8 thereon.
A light alloy is generally used and, in particular, an aluminum alloy is widely used as the structural material of the rotor 2, the rotor blades 4, the pump case 1, the stator blades 5, and so forth which form the vacuum pump, since the aluminum alloy is excellent in machining and can be precisely processed without difficulty. However, the hardness of aluminum alloy is relatively low as compared with other materials used for the structural material, and accordingly aluminum alloy may cause a creep fracture depending on the operating condition. Also, a brittle fracture may occur mainly caused by a stress concentration at the lower portion of the rotor 2, when the vacuum pump is in operation.
In the conventional vacuum pump having the above-described structure, when a brittle fracture occurs in the rotor 2 rotating at high-speed, for example, and a part of the rotor 2 crashes into the screw stator 7, since the screw stator 7 has an insufficient strength against a shock load caused by this crash, the screw stator 7 cannot absorb such a shock load and therefore radially crashes into a base member 1-1. Accordingly, this shock load produces a high rotating torque (hereinafter, referred to as xe2x80x9cdamaging torquexe2x80x9d) which causes the entire vacuum pump to rotate and which causes problems in that the entire pump case 1 is distorted, the fastening bolts 15 fastening the vacuum pump to the process chamber 17 are broken by this distortion torque, and the process chamber 17 is broken by the large damaging torque transferred thereto.
The present invention is made to solve the above-described problems. Accordingly, it is an object of the present invention to provide a vacuum pump which reduces a damaging torque produced when a rotor rotating at high-speed crashes into a screw stator or the like so as to prevent a damaging torque transferred to the process chamber or the like.
A vacuum pump according to the present invention comprises a rotor rotatably provided in a pump case; a plurality of rotor blades integrally provided with an outer circumferential surface of the upper part of the rotor; a plurality of stator blades positioned and arranged between the rotor blades; a screw stator arranged opposite to the outer circumferential surface of the lower portion of the rotor; and a rigid ring positioned and arranged at the outside the screw stator so as to be rotated by the shock load from the screw stator.
In the vacuum pump according to the present invention, when a brittle fracture occurs in the rotor rotating at high-speed, for example, and a part of the rotor crashes into the screw stator, a damaging torque causing the entire vacuum pump to rotate is likely to be generated. However, this damaging torque is absorbed by the rotation of the rigid ring and eventually subsides.
The vacuum pump according to the present invention may further comprise a buffer member between the screw stator and the rigid ring.
The vacuum pump according to the present invention may further comprise a low-frictional portion provided on at least one of the outer circumferential surface of the rigid ring and a surface opposite to the outer circumferential surface of the rigid ring so as to reduce the surface frictional force of the corresponding surface.
The vacuum pump according to the present invention may further comprise a buffer member between the screw stator and the rigid ring, and a low-frictional portion provided on at least one of the outer circumferential surface of the rigid ring and a surface opposite to said outer circumferential surface of the rigid ring so as to reduce the surface frictional force of the corresponding surface.
The vacuum pump according to the present invention may further comprise a base member, which serves as a base of the pump case and which is disposed on the outer circumferential surface of the rigid ring. Also, in this vacuum pump, a gap is provided between the base member and the rigid ring
The vacuum pump according to the present invention may further comprise a base member, which serves as a base of the pump case and which is disposed on the outer circumferential surface of the rigid ring, and a low-frictional portion is provided on a surface of the base member opposite to the outer circumferential surface of the rigid ring so as to reduce the surface frictional force of the surface opposite to the outer circumferential surface of the rigid ring.
The vacuum pump according to the present invention may further comprise a base member, which serves as a base of the pump case and which is disposed on the outer circumferential surface of the rigid ring, a gap is provided between the base member and the rigid ring and a low-frictional portion is provided on a surface of the base member opposite to the outer circumferential surface of the rigid ring so as to reduce the surface frictional force of the surface opposite to the outer circumferential surface of the rigid ring.
In the vacuum pump according to the present invention, the rigid ring is preferably composed of a metal selected from the group consisting of a titanium alloy, a nickel-chromium copper, a chromium-molybdenum steel, and a stainless steel.
In the vacuum pump according to the present invention, the buffer member may be provided with a plurality of hollows disposed along the rotating direction of the rotor.
In the vacuum pump according to the present invention, the buffer member may be provided with a plurality of hollows and hollow boundary portions alternately disposed along the rotating direction of the rotor, wherein each hollow boundary portion serves as the boundary between the adjacent hollows and is constructed so as to lean to a direction into which the hollow boundary portion is easily broken down by the shock load from the screw stator.
In the vacuum pump according to the present invention, the hollows provided in the buffer member are preferably crushed by the shock load when the shock load caused by the crash of the rotor into the screw stator is transferred to the buffer member.
In the vacuum pump according to the present invention, each hollow may have a parallelogram or diamond sectional shape.
In the vacuum pump according to the present invention, the low-frictional portion may adopt a structure in which a low-frictional surface treatment is applied to the surface whose frictional force is to be reduced or a low-frictional material is bonded to the surface.
In the vacuum pump according to the present invention, the low-frictional surface treatment is preferably performed by fluoroplastic coating, fluoroplastic-contained nickel plating, or fluoroplastic-impregnated ceramic coating.